1) Field of the Invention
The present invention relates to an image forming device such as a copying machine, a printer, or the like that records an image on a sheet of ordinary paper, more particularly to an image forming device that can be installed in multiple direction, for example, vertically or horizontally.
2) Description of the Related Art
Generally, in latent image type image forming devices such as electrophotographic recording devices to record an image on an ordinary paper, a visual image can be formed by forming an electrostatic latent image on a photosensitive drum, then supplying powder developer on the photosensitive drum by means of a developing unit, and developing the latent image on the photosensitive drum with the powder developer. Furthermore, the image of the powder developer on the photosensitive drum is transferred on a sheet of ordinary paper (recording paper). Then the sheet is separated from the photosensitive drum and then the image of the powder developer coated over the sheet is fixed on the sheet by means of a fixing unit.
In the developing process of the developing unit equipped with such an image forming device, the developer including carriers and toner or the developer including only toner is supplied to the photosensitive drum. When internal toner has been completely exhausted, the developing process cannot be continued. Hence it is needed to detect earlier the empty condition of the internal toner.
A prior art toner empty detecting mechanism will be explained by referring FIG. 23. As shown in FIG. 23, the developing unit has a toner detection sensor 91 mounted on the bottom of the toner hopper 90 holding toner. The toner detection sensor 91 produces a voltage according to the remaining amount of toner within the toner hopper 90.
A comparator 92 is arranged to compare a detection voltage with a constant slice voltage (threshold voltage). The comparator 92, as shown in FIG. 24, produces a toner empty signal when the toner detection sensor 91 outputs a detection voltage less than a slice voltage.
Where the developer includes magnetic toner, a permeability sensor, for example, is used as the toner detection sensor 91. The output voltage of the permeability sensor has the characteristic shown in FIG. 25. That is, in the case a small amount of toner (less than 1 g), the output voltage of the permeability sensor shows a linear characteristic proportional to the remaining amount of toner. Hence, when the amount of toner is small, the toner empty can be detected.
However, the prior art shown in FIGS. 23 to 25 has the following disadvantages:
The toner detection sensor 91 produces at most its output voltage of 5 volts, thus causing variations in voltage. Hence the method using a constant slice voltage as a reference value cannot detect the toner empty accurately.
Particularly, where the developing unit is detachable and attachable and the toner detection sensor 91 is mounted on the device body, the output voltage from the toner detection sensor 91 becomes smaller. As a result, the susceptible output voltage leads to an inaccurate toner empty detection.
The present applicant proposed a toner empty detecting method that can solve the above-mentioned problem in Japanese Patent Application No. 5-287654 (filed on Oct. 22, 1993 (Heisei 5)) or U.S. patent application No. 300,349, filed on Sept. 2, 1994. According to the patent application, the toner empty detecting method in which a control circuit receives the output from a toner detection sensor to detect the toner within a toner hopper is characterized by the steps of capturing the output from a toner detection sensor every fixed period; comparing the captured value with the average value of outputs from the toner detection sensor during the previous detection period; counting the times values captured during a predetermined detection period are smaller than the average value; calculating the average value of values captured during detection period; and producing an empty output when the count value obtained by comparing the calculated value with a predetermined value is larger than the predetermined value.
The above empty detecting method has the following advantages.
Adverse effect due to variations in voltage of the toner detection sensor can be removed by comparing the output value from the toner detection sensor with the average value during the previous detection period and then detecting the toner empty condition to a relative level.
When the toner empty condition is detected using only the result obtained by comparing the output value of the toner detection sensor with the average value, the toner empty condition may be erroneously detected with a small output value from the toner detection sensor because of a variation in the amount of toner associated with the rotation of the agitator (a supply roller) within the developing unit. However, the above-mentioned empty detecting method counts the times captured value is smaller than an average value during a predetermined detection period, compares the count value with the predetermined value, and produces an empty output when the count value is larger than the predetermined value. As a result, the toner empty condition can be accurately detected even to toner agitated and moved.
Changing the subject, the present inventors have vigorously studied an image forming device (Japanese Patent Application No. 5-261604 (filed on Sept. 24, 1993 (Heisei 5)) or U.S. patent application No. 220,205, filed on Mar. 3, 1994) that can form an image with no difference in quality in the horizontal and vertical installations. As a result, the following problems have been found.
As to the toner empty control, in two cases of the device body horizontally installed and the device body vertically installed, the empty output generation timing changes when an empty control is performed under a common decision condition. Hence a stable empty detection cannot be accomplished.
In the image forming step in which a sheet passes through the fixing unit, moisture contained in the sheet evaporated within the image forming device affects adversely the photosensitive drum. Particularly, the current image forming device has the structure that the fixing unit is placed above the photosensitive drum when the device body is installed vertically. Hence, the moisture effect is remarkable, compared with the horizontal installation. Ordinarily, the image forming device includes a fan to exhaust heat out of the device and cool the inside of the device. However, in the vertical or horizontal installation, since heat current with vapor ascends differently, it is difficult to cool stably the inside of the device and to exhaust stably vapor. Where the temperature in the process unit including a photosensitive drum is low at a power-on time, the vapor moisture tends to condense on the process unit, thus having significant effect on the process unit. For example, condensed moisture sweated over the photosensitive drum hastens the degradation of the photosensitive drum, thus shortening its operational life.
Usually, the device installed vertically occupies a space (a device occupying area) smaller than the device installed horizontally. Hence the device is installed vertically in most cases for space saving. However, the vertical installation has a disadvantage in stacking printed sheets. Actually the vertical installation occupies nearly the same space as that for the horizontal installation. This inconvenience cancels the merit of the vertical installation. Since printed sheets stacked on the printed surface thereof, the printed sheets must be rearranged in serial number after the printing operation, whereby this work is troublesome.